Non-invasive biosensing technologies have enormous potential for applications ranging from athletics, to neonatology, to pharmacological monitoring, to personal digital health. Among the biofluids potentially used for physiological monitoring (e.g., sweat, blood, urine, saliva, tears), sweat has arguably the least predictable sampling rate in the absence of technological solutions. An excellent summary is provided by Sonner, et al. in the 2015 article titled “The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications,” Biomicrofluidics 9, 031301, included by reference herein in its entirety. However, with proper application of technology, sweat can be made to outperform other non-invasive or less invasive biofluids in predictable sampling. As a result, sweat ducts can provide a route of access to many of the same biomarkers, chemicals, or solutes that are carried in blood and can provide significant information enabling diagnosis of ailments, health status, toxins, performance, and other physiological attributes even in advance of any physical sign.
However, one challenge to creating accurate sweat sensing devices is developing sensors that are practical for use in a wearable, semi-disposable device, when the required sensitivity and specificity of such sensors is typically achieved in expensive bench-top configurations that are afforded stable operating environments. Sweat sensors, however, must work within a device that is sufficiently small and robust to be worn on the body. These devices must operate in a dynamic environment, and be inexpensive enough to be periodically replaced. When bench-top ion selective electrodes (“ISE”), and their accompanying reference electrodes, are moved from the laboratory to a sweat sensor, they must be greatly reduced in size, are often deposited on a flexible substrate, and exposed to skin. However, such configurations are prone to drift due to their small size, may experience interference from the body, may delaminate from the substrate, or be abraded through contact with skin or other device components. The Horiba LaquaTwin, covered by U.S. Pat. No. 5,200,706, successfully incorporates ISEs into a hand-held device for water sampling. However, these sensors are still too large and expensive for use in wearable sweat sensors.
What is needed are simple, yet robust methods to incorporate ISEs and reference electrodes into wearable configurations that are free from drift, delamination, and abrasion over the use period of a sweat sensing device. In particular, sweat sensing devices hold tremendous promise for use in workplace safety, athletic, and military settings. One potential application that can improve personal safety and performance in these settings, is a robust, wearable sweat electrolyte loss monitor. As disclosed herein, a sweat sensing device is configured to monitor sweat electrolyte concentrations, ratios and trends under demanding use conditions.